The present invention relates to a process for grinding a plastic material to produce superfine particles and compositions, such as suspensions and dry powders, resulting from such process. Common ice is the abrasive for grinding.
Plastics are commonly used for a seemingly unlimited number of modern products. For certain uses, plastics must be finely ground. A common way to grind plastics is to mix them with powdered dry ice and pass the mixture through a milling apparatus with a fine grating on the bottom that allows particles small enough to pass through. Though this process works, it is very tedious and takes a long time to even get a small amount of product. Cleaning the mill is also laborious, not to mention the safety hazard of milling fine materials in the air.
Milling media commonly include sand, spheres of silica, stainless steel, silicon carbide, glass, zirconium, zirconium oxide, alumina, titanium, polymeric media, or inorganic salts such as sodium chloride, for example. U.S. Pat. No. 3,642,686 relates to pigment preparations made by using an inorganic salt as a grinding agent. In one example, a kneaded composition is decomposed by adding 50 parts ice and isopropanol, and is granulated by further kneading, followed by wet grinding with about 3000 parts water in a mill to form a very fine granulate. U.S. Pat. No. 3,745,722 relates to the finishing of workpieces using abrasive grains in an ice matrix. GB 1 588 777 relates to solid pigment formulations that may be manufactured in a kneader, and wherein sodium chloride is optionally used as a grinding auxiliary. Kneading compositions may be broken up by adding saturated sodium chloride and ice for grinding. EP 0068684 relates to a process for preparing a particulate gel where a swollen gel is subjected to shear stress to produce particles. Parameters varied included ratio of polymer:ice:water, time of liquidizing, blade speeds, and xe2x80x9cscale-up,xe2x80x9d however, the particles obtained from these conditions are large (e.g.,  greater than 500 microns). In order to obtain a fraction of particles smaller than 200 microns, use of mesh was required. U.S. Pat. No. 5,520,888 relates to treating biomedical waste with ozone containing ice where entrapped ozone is released upon melting of the ice. Particles of about xc2xd to xc2xc inch are obtained.
Obtaining finely ground particles in the range of or below 100 microns is technically difficult, since plastic particles can be elastic and will distort rather than fracture during grinding. Also, certain plastics are difficult to grind because they fuse rather than breaking up into fine particles. For example, attempts at producing an artificial latex of a polyhydroxyalkanoate (PHA) copolymer termed NODAX(trademark), The Procter and Gamble Company, have included emulsifying a solution in an organic solvent and then stripping off the solvent to leave solid particles suspended in water. These process conditions are difficult to control and often inefficient, leaving a lot of wasteful coagulum. The requirement for a large amount of surfactants needed for emulsifying a NODAX(trademark) solution is also a concern. Direct cryogenic dry grinding of NODAX(trademark) resin using dry ice as a coolant requires no lingering additives like surfactants. Unfortunately, this method had limited success in creating a fine powder of NODAX(trademark). The particle size could not be well controlled, and amalgamation of particles to form gritty aggregates became a problem.
Individual PHA granules found in biological cells as inclusion bodies such as those described in U.S. Pat. No. 5,849,854 to Noda are up to 1 micron in size or even smaller. Biological cells containing PHA or a mixture of biomass and PHA granules can aggregate to be about 40 microns in size, however, the individual PHA granules inside of this aggregate are still 1 micron or less. The 40 micron aggregates are not pure PHA granules; they contain well over 20% to as high as 80% biomass.
The present invention addresses problems in the art of methods for grinding plastics and provides improvements herein. A method that is inexpensive, simple to operate, fast, energy efficient, and that requires no major equipment is provided herein.
The present invention provides a process for producing particles of a plastic material having a glass transition temperature of between xe2x88x9220xc2x0 C. to 120xc2x0 C. The process comprises grinding the plastic material in the presence of an abrasive consisting essentially of ice at a temperature of melting ice, the grinding for a time sufficient to produce a suspension of particles having an average diameter of 10 microns to 200 microns, or less than 100 microns, or about 10 to 40 microns for certain plastics. During the grinding process, the ice melts to form a slurry.
Resulting suspensions in water or dried particles therefrom in the form of a powder are useful for a variety of applications such as, for example, artificial latex, coatings such as for paper, binders, additives for paints, adhesives, drug tablets, fertilizer pellets, a carrier for drugs or dyes or volatile actives such as perfumes, repulping-friendly xerographic toners, as well as cosmetic, laundry, or food applications.
The grinding method of the present invention uses inexpensive and readily available common ice as the abrasive. The method is simple to operate, fast, energy efficient, and requires no major equipment. The grinding method of the present invention is essentially free from typical contaminants found in artificial latex, such as surfactants or residual solvents, since the present process does not require organic solvents to dissolve the polymer as practiced in a conventional artificial latex production. Neither does the grinding method strictly require use of expensive and hazardous gas-evolving cryogens like dry ice or liquid nitrogen. Since the process does not create airborne dust during grinding, the process is free from explosion hazard and is readily contained. Since the grinding medium is regular ice, contamination of the product by chipped or crushed grinding media common to media mills is not present.
PHA powders having an average particle diameter of 10 microns to 100 microns and having greater than 80% purity are further embodiments of the present invention. PLA powders having an average particle diameter of 25 microns to 100 microns are further embodiments of the present invention.